Optical fibers whose cores have noncircular cross sections are used, for example, in sensors and couplers of various types, in polarization-maintaining fibers for coherent optical communication systems, and are of interest because such systems allow higher spacing between repeaters than other kinds of fibers.
Heretofore, fibers with noncircular core cross section could be obtained, in the case of simple geometrical shapes such as elliptical core fibers, by mechanical deformation of a preform with an originally circular core. Mechanical deformation, however, can give rise to problems of mechanical strength and constancy of optical characteristics of the fiber with time or upon subjection of the fiber to various thermal effects.
In the case of more complex shapes and for multicore fibers, where the noncircular core is produced without mechanical deformation or in conjunction with mechanical deformation, the apparatus which has been required to fabricate such fibers has been very complex. For example, the apparatus may require a drawing furnace divided into a plurality of thermally insulated sections with independently adjustable temperature (see the paper entitled Coupled/Noncoupled Wave Transmission in Long-Length Multicore Optical Fibers, R. S. Romanyuk and J. Dorosz, ECOC '84, Stuttgart, Sept. 3-6 1984).